Fassi was the Italian singles champion from 1943 to 1954, won a bronze medal at the world championship in 1953, and clinched gold medals at the European championship in 1953 and 1954. He also competed in the 1952 Winter Olympics in Oslo, Norway, finishing sixth. In 1961, following a plane crash in which many American skaters and coaches were killed, Fassi was invited to the United States to help rebuild the skating program. His pupil Peggy Fleming won a gold medal at the 1968 Olympics in Grenoble, France, and Fassi’s coaching success continued with Dorothy Hamill and Britain’s John Curry, both of whom won gold medals at the 1976 Winter Games in Innsbruck, Austria. Fassi went on to nurture Robin Cousins to a gold medal at the 1980 Olympics in Lake Placid, New York, and to coach athletes who won a total of four world championships and national titles in 15 countries.

Fassi partnered with his wife, Christa, for most of his coaching career; they were famous for their ability to develop the character of their skaters as well as their physical ability. The last world champion Fassi coached was American Jill Trenary, who won her gold medal in 1990. Fassi was with two of his skaters, American Nicole Bobek and Romanian Cornel Gheorge, at the 1997 World Championships in Lausanne, Switzerland, when he had a fatal heart attack. He coauthored (with Gregory Smith) Figure Skating with Carlo Fassi (1980).